Method and apparatus for enhancing flame radiation in a coal-burner retrofit

ABSTRACT

An apparatus for retrofitting a coal-fueled burner of, e.g., a furnace, with a gas-fueled burner system for enhancing flame radiation of a gas flame. The gas-fueled burner system includes a flame charging system and an electrically isolated electrode. A time-varying voltage is applied to the flame charging system and the flame charging system imparts a corresponding time-varying charge onto the flame. The flame responds to the time-varying charge by increasing its luminosity and emissivity.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. application Ser. No. 13/729,159, filed Dec. 28, 2012 (matter number 2651-033-03), which claims priority benefit from U.S. Provisional Patent Application No. 61/582,239, filed Dec. 30, 2011; each of which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

BACKGROUND

In some furnace and burner designs, it may be desirable to transfer at least a portion of combustion energy as radiated energy. However, some types of flames are poorly radiating. In some cases, heat is radiated by flame impinging on a higher-emissivity refractory surface. However, this is not possible in all furnaces. Even when refractory walls can be used, hard refractory walls add weight and cost to furnace installations.

Fuels with a relatively high carbon to hydrogen (C/H) atomic ratio, e.g., heavy fuel oils and coal, may be used to produce flames naturally having relatively high emissivity. However, these fuels are also prone to higher particulate and carbon monoxide (CO) emissions.

Cleaner burning fuels such as natural gas exhibit relatively poor heat transfer via thermal radiation owing to low emissivity of their flames. In particular, flames from such clean burning fuels tend to be non-luminous, non-incandescent, and produce relatively low radiant energy in furnace conditions. In contrast, flames produced from coal powder generally include solid particles of carbon from decomposition and cracking of complex hydrocarbons. These carbon particles incandesce and radiate at flame temperatures.

Due to the above-mentioned inherent high emissivity of flames from combustion of heavy fuel oils or coal, boilers and furnaces originally designed for coal or heavy fuel oils require significant reconfiguration in order to make effective use of low-emissivity fuels such as natural gas. For example, a coal-burning furnace system may provide a balance of heating between radiant and convection sections of steam tubes in conjunction with higher-emissivity refractory walls to direct thermal energy. Converting such coal-burning system to use natural gas, with its attendant low-emissivity flames, conventionally includes changes to the furnace structure itself to compensate for the low emissivity of gas-fueled flames. Such changes often include modification or replacement of existing forced draft fans, air heater modifications, and superheat and reheat surface modification.

What is needed is a technology that can transform a poorly radiating flame into a highly radiating flame. Better radiant heat transfer can reduce the size of a furnace. Furnace size is a significant component of overall reactor or heater cost. Such a technology could reduce the overall size, weight, and cost of new furnaces and increase the throughput of existing furnaces and processes driven by furnaces. Additionally, such a technology would desirably be switchable to allow for rapid heating and cooling cycles not possible with designs having high thermal mass. Moreover, such a technology would desirably offer directed radiation difficult or impossible to achieve with high thermal mass, intermediate radiator approaches. Still further, better radiant heat transfer technology at the burner would permit fuel conversion for furnaces originally designed for fuels that naturally produce high-emissivity flames.

SUMMARY

It was found in laboratory testing that the application of alternating electrical energy to a low emissivity flame greatly increases flame emissivity.

According to an embodiment, a system for radiating energy from a flame, such as a hydrocarbon flame, may include a flame charging system configured to receive a time-varying voltage and impart a corresponding time-varying charge or voltage onto the flame. The flame charging system may have at least intermittent contact with the flame, and may be embodied as a portion of a fuel nozzle, flame holder, or discrete electrode past which the flame is directed, may include an ion-ejecting electrode, or may include an ionizer. An electrically isolated conductor may be located proximate the flame. The electrically isolated conductor may be arranged to be in electromagnetic communication with the time-varying charge imparted onto the flame, and may be configured to interact with the time-varying charge of the flame to increase radiated thermal energy.

According to another embodiment, a method for radiating energy from a hydrocarbon flame may include providing a hydrocarbon fuel, igniting the hydrocarbon fuel to produce a flame, energizing the flame with a time-varying voltage or charge, and supporting an isolated electrical conductor adjacent to the flame to cause the flame to emit enhanced visible or infrared light energy. The electrically isolated conductor may be arranged to be in electromagnetic communication with the time-varying voltage or charge imparted onto the flame to cause the increased radiated thermal energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a system for radiating energy from a flame, according to an embodiment.

FIG. 2 is a diagram illustrating the system of FIG. 1 in relation to a system including a heat transfer surface, according to an embodiment.

FIG. 3 is a flow chart showing a method for increasing radiation from a flame, according to an embodiment.

FIG. 4 is a diagram illustrating a theory explaining the behavior of the methods and systems described in conjunction with FIGS. 1-3, according to an embodiment.

FIG. 5 is a diagram illustrating a conventional coal-fired furnace.

FIG. 6 is a diagram illustrating a retrofit of the furnace in FIG. 5 with a gas burner having the system for radiating energy from a flame, according to an embodiment.

FIG. 7 is a flow chart showing a method for retrofitting a coal-fired furnace with the system for radiating energy from a flame, according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

FIG. 1 is a diagram illustrating a system 101 for radiating energy from a flame 102, according to an embodiment. In the system 101, a flame charging system 104 may be configured to receive a time-varying voltage and impart a corresponding time-varying charge or voltage onto a hydrocarbon flame 102. An electrically isolated conductor 106 proximate the flame 102 was found to interact with the time-varying charge of the flame 102 to cause the flame 102 to change in appearance from being substantially transparent to being bright yellow. It was concluded that the electrically isolated conductor 106 may be arranged to be in electrical communication with the time-varying charge imparted onto the hydrocarbon flame 102. The change in flame appearance was believed to correspond to an increase in emissivity of heated species within the flame.

The flame charging system 104 may include a flame energization electrode having at least intermittent contact with the flame 102. The flame charging system 104 may be configured to receive a time-varying voltage selected to cause the flame energization electrode to impart the corresponding time-varying electrical charge or voltage onto the flame. An energization electrode may operate by conduction of voltage from the flame energization electrode to the flame.

Additionally or alternatively, the flame charging system may include a charge-ejecting electrode disposed proximate to the flame. The charge-ejecting electrode may be configured to receive a time-varying voltage and to eject a corresponding time-varying electrical charge toward the flame. The charge-ejecting electrode may be referred to as a corona electrode. The charge-ejecting electrode may include a sharp-tipped electrode.

Additionally or alternatively, the flame charging system may include an ionizer configured to receive a time-varying voltage and provide a fluid medium carrying corresponding time-varying electrical charge or voltage to or in proximity to the flame. For example, the ionizer may be configured to impart the time-varying electrical charges onto a fuel. Additionally or alternatively, the ionizer may be configured to impart the time-varying electrical charges onto combustion air. Additionally or alternatively, the ionizer may be configured to impart the time-varying electrical charges onto one or more types of charge carriers and to deliver the one or more types of charge carriers to the combustion reaction.

According to an interpretation, a phase-varying electrical energy interchange between the time-varying charge of the flame 102 and the electrically isolated conductor 106 may correspond to an increase in the formation of carbon molecules or carbon agglomerations in the flame 102. The carbon molecules or carbon agglomerations in the flame may incandesce and increase the emissivity of the flame. No increase in soot output from the flame 102 was observed in experiments. According to embodiments, the resultant increase in radiation from the flame 102 may be used to increase radiation heat transfer to an apparatus or workpiece, e.g., one or more steam tubes in a radiant heating volume of a furnace (see, e.g., FIGS. 5A-5B).

The system 101 may further include a flame holder 108 configured to anchor the flame 102. The flame holder 108 may be electrically isolated or an electrical insulator. A fuel source 110 such as a hydrocarbon gas fuel source including a nozzle or hole 112 may be configured to stream the hydrocarbon gas past the flame holder 108. The hydrocarbon gas fuel source may include an electrically insulating pipe or hose 114 configured to electrically isolate the nozzle or hole 112 from a relative ground.

A time-varying voltage source 116 may provide a modulated voltage to the flame charging system 104. For example, the time-varying voltage source 116 may include a microcontroller, field-programmable gate array (FPGA), application specific integrated circuit (ASIC), state machine, integrated circuit(s), and/or discrete circuitry to output a waveform. The time-varying voltage source 116 may further be configured to select the waveform responsive to open-loop logic and/or feedback from a sensor circuit (not shown). The time-varying voltage source 116 may further include an amplifier configured to receive the waveform and output the time-varying voltage. The flame charging system may include a flame energization electrode 104 arranged to be in substantially continuous contact with the flame 102 when the flame 102 is burning. The time varying voltage and a geometry of the flame energization electrode 104 may be selected to substantially prevent formation of an electrical arc.

The time-varying voltage may be selected to cause a phase-varying electrical energy interchange between the flame 102 and the electrically isolated conductor 106. The electrically isolated conductor 106 may be arranged in a capacitive relationship with the time-varying charge imparted onto the flame. For example, the time-varying voltage may be selected to cause a phase-varying capacitive energy storage between the flame 102 and the electrically isolated conductor 106. Additionally or alternatively, the electrically isolated conductor 106 may be arranged in an inductive relationship with the time-varying charge imparted onto the flame 102. For example, the time-varying voltage may be selected to cause a phase-varying inductive energy storage or a combined inductive-capacitive energy storage between the flame 102 and the electrically isolated conductor 106.

Additionally or alternatively, the time-varying voltage may be selected to cause a phase-varying local transition state concentration and/or charge balance in the flame. According to an embodiment, the phase-varying electrical energy interchange or phase-varying local transition state concentration and/or charge balance may correspond to a decrease in an average flame temperature. The decrease in average flame temperature may be viewed as an outcome of a time-varying rate of reaction and/or as an outcome of radiating more energy from the flame 102.

Additionally or alternatively, the time-varying voltage may be selected to cause a phase-varying flame location having a corresponding phase-varying ratio of reactants (e.g., as a phase-varying oxygen concentration). By time-varying a reactant ratio, the combustion reaction may be induced to selectively and transiently output soot sufficient to incandesce and radiate, but subsequently fully oxidize to provide clean combustion.

The electrically isolated conductor 106 operates as described when configured as a steel ring. According to embodiments, the electrically isolated conductor may include a ring or ring segment at least partially surrounding the flame. The electrically isolated conductor 106 may be arranged to be substantially coaxial to the flame.

The electrically isolated conductor 106 operates as described when in substantially continuous physical contact with the flame. According to another embodiment, the electrically isolated conductor 106 may occasionally or intermittently come into physical contact with the flame. “Physical contact” may be defined as visible contact with a flame edge.

An arrangement corresponding to 101 may use a hydrocarbon gas flame 102 produced by combustion of propane. Other fuels may alternatively or additionally be burned and/or other reduction-oxidation reactions may be supported to operate as described. For example, the hydrocarbon may include greater than or fewer than three carbon atoms. Other illustrative hydrocarbon fuels may include natural gas, ethane, butane, liquefied petroleum gases, refinery gas or liquid mixtures, gasoline, diesel, fuel oil, coal, etc.

FIG. 2 is a diagram illustrating the system 101 of FIG. 1 in relationship to a system 201 including a heat transfer surface 202, according to an embodiment. Accordingly, embodiments may include a surface 202 configured to receive radiant energy from the flame 102. For example, the surface 202 may comprise a portion of an industrial process 201 configured to receive radiant energy from the flame 102, a heating system 201 configured to receive radiant energy from the flame 102, an electrical power generation system 201 configured to receive radiant energy from the flame 102, a land vehicle, watercraft, or aircraft including an apparatus 201 configured to receive radiant energy from the flame 102, and/or a structure (not shown) configured to hold a workpiece 202 to receive radiant energy from the flame 102. In one example, the heat transfer surface 202 may include one or more steam tubes.

FIG. 3 is a flow chart showing a method 301 for increasing radiation from a flame, according to an embodiment. Beginning at step 302, a fuel may be provided. For example, providing a fuel may include providing a hydrocarbon fuel. Such a hydrocarbon fuel may have one to three carbon atoms per molecule, or may have more atoms per molecule. While various embodiments may include increasing radiation output of flames produced by combusting other fuels, low molecular weight hydrocarbon gas fuels are illustratively addressed because such fuels typically produce flames that are substantially transparent, owing to low emissivity of the gas and the reaction intermediates, and thus may particularly benefit from methods described herein. According to embodiments, the method 301 may be used to increase thermal radiation from a natural gas flame.

Proceeding to step 304, the hydrocarbon fuel may be ignited to produce a flame. The method 301 may include premixing air or other oxidizer and the fuel (not shown). In some embodiments, the flame may include or be a diffusion flame.

In step 306, the flame may be energized with a time-varying voltage or electrical charge. Energizing the flame with a time-varying voltage or electrical charge may include driving a first electrode near or at least partially in the flame with a corresponding time varying voltage. According to embodiments, energizing the flame with a time-varying voltage or electrical charge may include driving a fuel nozzle or a flame holder with a corresponding time varying voltage. Additionally or alternatively, energizing the flame with a time-varying voltage or electrical charge may include driving an ionizer with the time-varying voltage to create the corresponding time-varying electrical charge. Additionally or alternatively, energizing the flame with a time-varying voltage or electrical charge may include driving an ion-ejecting electrode with the time-varying voltage to eject ions corresponding time-varying voltage or electrical charge toward or onto the flame.

Various voltage waveforms, amplitudes, and frequencies were used in experiments, and others have been hypothesized. A relatively wide range and combination of parameters may be used to increase radiation emissions from the flame. According to embodiments, energizing the flame with a time-varying voltage may include energizing the flame with a periodically-varying voltage at 50 to 10,000 hertz frequency. For example, the flame may be energized with a periodically-varying voltage at a 50-1000 hertz frequency. During experiments a 400 hertz frequency resulted in a larger amount of radiated energy than did a 50 hertz frequency, other parameters being equal.

Waveforms may similarly be varied. For example, energizing the flame with a time-varying voltage may include energizing the flame with a square wave, sawtooth wave, sine wave or other waveform. It was noted during experiments that a square wave resulted in a larger shift to radiated energy than did a sinusoidal waveform, other parameters being equal.

Similarly, voltage and geometry may be varied. According to embodiments, energizing the flame with a time-varying voltage may include energizing the flame with a 1000 volt to 115,000 volt amplitude. For example, the voltage may be between 8000 to 40,000 volt amplitude. It is believed that larger flames may respond favorably to larger voltages.

Step 306 may further include providing a flame energization geometry or control circuitry to substantially prevent arcing. For example, the flame energization voltage may be alternated or applied in such a way as to not exceed the breakdown voltage of the ambient environment or the flame. Exceeding the breakdown voltage will produce an electrical spark in a phenomenon known as arcing. One approach for reducing arcing may be to smooth all edges of the first electrode to avoid charge concentrations that may tend to initiate an arc. Another approach may be to control voltage with sufficient accuracy to avoid voltage spikes that may initiate an arc. Another approach may be to use a feedback circuit in combination with a current limiting power supply to cut power upon sensing arcing or incipient arcing conditions.

Proceeding to step 308, an electrical conductor may be supported adjacent to the gas flame to cause the flame to emit enhanced visible and/or infrared light energy. An example electrical conductor 106 may be seen in FIG. 1. The electrical conductor may, for example, be in electrical continuity with ground through a resistance greater than about one mega-ohm and/or may be insulated or isolated from ground. Use of a high resistance to ground and/or isolation of the electrical conductor may allow the electrical conductor to electrically float.

Various theories may help explain the behavior described herein. For example, the electrical conductor may be in capacitive communication with the energized flame. Alternatively or additionally, the electrical conductor may be in inductive communication with the energized flame. The flame emission behavior described herein may involve a periodic energy exchange between capacitance and/or inductance and thermal energy of the flame. Additionally or alternatively, the electrical conductor may operate in combination with the modulated, time-varying charge on the flame to reduce the concentration of a transition state due to removal of one sign of charge during one half-cycle, and then act as a source of some or all of the sign of charge and local reaction transition state concentration during the subsequent half-cycle. Since soot is electrically conductive and soot particles can concentrate electrical fields, an external electrical field may increase the precipitation of soot from a flame. Ionic mechanisms of soot formation have been postulated in the literature, but no introduction of external electromagnetic fields has been previously suggested. According to embodiments, the time-varying voltage may be selected to cause an increase in an incandescing soot fraction of the flame.

An explanation of these alternative or complementary theories may be understood by reference to FIG. 4, below.

According to embodiments, the electrical conductor may include a ring surrounding an upper portion of the flame and not in contact with the flame.

Responsive to one or more interactions between the electrical conductor adjacent to the flame and the flame energization electrode, the flame may emit enhanced visible and/or infrared light energy, shown as step 310. According to one explanation, interactions between the charge on the flame and the conductor may cause the flame to emit enhanced visible or infrared light energy responsive to increasing the emissivity of reaction products and reaction intermediates in the flame. For example, increasing radiation from the flame may include shifting a reaction path to at least temporarily produce soot. The soot may emit black body radiation corresponding to the flame temperature.

Proceeding to step 312, at least a portion of the radiated energy may be transmitted at to an apparatus.

FIG. 4 is a diagram 401 illustrating a theory explaining the behavior of the methods and systems described in conjunction with FIGS. 1-3, according to an illustrative embodiment. In the diagram 401, voltage, V, is plotted as a function of time, t. A first voltage waveform 402, shown as a solid line approximating a sine wave, may correspond to a time-varying voltage applied to the first electrode described above. When the conductor is allowed to float, its voltage may be described by a phase-shifted waveform 404, shown as a dashed line. As a voltage 402 applied to the first electrode increases, the voltage of the conductor 404 may follow.

During a first half cycle 406 of the system, the voltage applied to the flame 402 may be lower than the voltage 404 responsively held by the conductor. During the half cycle 406, electrons may be attracted out of at least portions of the flame toward the conductor. Similarly, positively charged species may be attracted due to proximity of the conductor to the flame. Because the charge to mass ratio of electrons is so much larger than the charge to mass ratio of positive species present in the flame, the movement of electrons may be responsible for most or substantially all of the effects described herein. The effect of the attraction of electrons out of the flame may be viewed in several ways. Remaining positive charges may unbalance the local population of transition states (excited molecules and intermediates) or charges. The positive charge imbalance may tend to be associated with carbon molecules or agglomerations, which hold heat produced during the previous half-cycle, and emit the heat as radiation. According to a second view, some of the energy of the system may be temporarily converted to a capacitive and/or inductive energy held in a field between the flame and the conductor.

During a second half cycle 408 of the system, the voltage applied to the flame 402 may be higher than the voltage 404 responsively held by the conductor. During the half cycle 408, electrons may be attracted from proximity to the conductor and into the flame. During the second half cycle 408, due to the concentration of transition states and/or the charge balance the combustion reaction may again be satisfied, causing carbon molecules or agglomerations to be consumed. According to the second view, energy may be extracted from a capacitive and/or inductive energy field to be expressed as heat energy in the flame.

Other theories may also explain the effects described herein. For example, it is possible that an increased rate of reaction is provided simply by mixing forces as charged species stream past and collide with complementary species. A reduced rate of reaction may then be seen during portions of the cycle where the reactant velocities stagnate and reverse direction.

Notwithstanding particular mechanisms which may cause the described behavior, the behavior described and claimed herein was observed experimentally, as may be illustrated by the following example(s).

EXAMPLES Example 1

Referring to FIG. 1, in a control experimental apparatus variant that did not include the conductor 106, a propane gas flame continued to burn substantially transparently when a voltage was applied to the energization electrode 104.

Geometry:

-   -   Energization Electrode 104:     -   A 3-inch nominal diameter steel pipe was cut to a length of 3¾         inches. The energization electrode 104 was positioned about 16         inches above a 0.775-inch diameter hole 112.     -   Conductor 106:     -   Absent.     -   Fuel Source 110:     -   A 0.775-inch diameter hole 112 was formed in a threaded ¾-inch         steel pipe end. The threaded steel end was mounted on piece of         ¾-inch steel pipe about 8 inches in length. A non-conductive         hose 114 was secured to an upstream end of the fuel pipe 110.         Propane was supplied at a pressure of about 8 PSIG.

Energization:

-   -   A time-varying voltage was applied as a square wave at a         frequency of 50-1000 Hz. An indicated voltage of 2-8V was         indicated by a National Instruments PXI-5412 waveform generator         mounted in a National Instruments NI PXIe-1062Q chassis. The         waveform was amplified 4000× by a TREK Model 40/15 high voltage         amplifier to produce a time-varying relative driving voltage         range of 8000 V to 32000 V at the energization electrode 104.     -   Observations:

There was no visible flame difference responsive to the applied time-varying voltage.

Example 2

Referring again to FIG. 1, an experimental apparatus 101 included an ungrounded 6 inches steel pipe flange as the conductor 106. The pipe flange 106 was supported by refractory bricks concentric to and at a height of 8 inches above the bottom edge of the energization electrode 104.

The energization electrode 104 was again energized according to the parameters given above.

The apparatus 101 produced a much yellower and surging flame. The brightness of the light output was greater when the energization electrode 104 was driven with a square wave at 1000 Hz than a square wave driven at the same voltage at 50 Hz.

The gap between the top of the energization electrode 104 and the bottom of the ring 106 was 4¼″ axially. Adding a second ring 106 on top of the first ring 106 gave no noticeable increase in brightness. If anything, adding a second ring diminished the brightness somewhat.

Blue tendrils were noted between the hole 112 and the flame holder 108 when a voltage waveform was applied to the energization electrode 104 in the presence of the ring 106. No blue tendrils were seen when voltage was applied in the absence of the ring 106. Electrical isolation of the pipe 110 from ground was measured. Some leakage to ground was found, but very little. The inventors believe that the observed blue tendrils may relate to a mechanism whereby electrically induced plasma channels tend to generate charged particles and/or radicals that operate as combustion initiators.

Example 3

The apparatus of EXAMPLE 2 was modified by grounding the ring 106. Upon application of the energization voltage, a very brief increase in flame luminosity was noted. The flame did not exhibit any sustained increase in luminosity.

FIG. 5 is a diagram illustrating, in part, a conventional coal-fired furnace 500. The coal-fired furnace 500 includes a burner system 501. The burner system 501 includes a pulverized coal burner 503 that supports a flame 505. The burner system 501 also includes a pulverized coal and combustion air delivery device 507. The pulverized coal burner 503 is configured to receive pulverized coal as a combustion fuel and combustion air from the pulverized coal and combustion air delivery device 507. The pulverized coal burner 503 or the delivery device 507 includes one or more nozzles (not shown) for delivery of pulverized coal to the pulverized coal burner 503 from a coal source, such as a hopper and/or pulverizer (not shown). In some examples, the delivery device 507 may be considered as including a blower or fan to entrain and convey the pulverized coal to the nozzle(s). Combustion air for the pulverized coal burner 503 may be delivered via the delivery device 507 in a manner suitable for mixture with and combustion of the pulverized coal.

The coal-fired furnace 500 includes a furnace wall 510 having a top end 512 and a bottom end 514. The coal-fired furnace 500 may include steam tubes 516 disposed about an interior of the furnace wall 510, arranged to receive radiant thermal energy from the flame 505 and combustion products resulting from the flame 505. As discussed above, coal has a relatively high C/H ratio, and the pulverized-coal burner 503 thus produces as a combustion product a significant amount of particulate, which may at least contribute to radiant heat transfer to the steam tubes 516. In addition to thermal energy, combustion product may include ash, soot, and/or gases (e.g., carbon monoxide), which may exit the furnace wall 510—for gases and lightweight particles—at the top end 512 via a flue 520 or—for heavier products such as “bottom” ash—may exit 522 the furnace wall 510 at the bottom end 514.

It will be recognized that the coal-fired furnace 500 of FIG. 5 is a generalized and simplified illustration and thus lacks detail and scale. Some implementations of a coal-fired furnace 500 may include multiple burner systems 501 placed at various elevations and/or geometries about the furnace wall. The illustration of FIG. 5 is provided merely to provide a base system from which the burner system 501 may be removed and replaced.

FIG. 6 is a diagram illustrating a retrofit of the furnace in FIG. 5 with a gas burner system 601 including system for radiating energy from a flame. A portion of a furnace 600 includes the replacement burner system 601 (corresponding to the system 101 for radiating energy from a flame, described above). The replacement burner system 601 replaces the coal-oriented burner system 501 (shown in dotted lines). The replacement burner system 601 may include a flame charging system, a gas burner 602 and a gas and combustion air delivery device 606. Although not illustrated in FIG. 6, the gas burner 602 and/or gas and combustion air deliver device 606 may include a flame holder, a gas fuel source having at least one nozzle or hole for delivering a gaseous fuel, such as natural gas, and a combustion air delivery device (not illustrated above). The burner system 601 may include, at the gas burner 602, the specific elements described above for the system 101 for radiating energy from a flame. Specifically, the gas burner 602 may include the flame holder 108, a gas fuel source 110 having one or more nozzles 112, a flame charging system 104, and an electrically isolated electrode 616 (corresponding to, e.g., 106 of FIG. 1) that operate as described above. In some embodiments, the flamer holder may operate as an electrode, as described above.

As FIG. 6 is directed to a replacement burner system 601, with the flame charging system enhancing radiative properties of the flame, the remaining furnace elements may remain relatively unmodified. That is, the furnace 600 may include the furnace wall 510, shown in partial section, having the top end 512 and the bottom end 514. Due to the flame charging system, the flame 604 may provide thermal energy via radiation to the steam tubes 516 (as an example of a heat transfer surface 202 described above) to heat water or water vapor contained therein for steam. The steam may be utilized for power generation, HVAC, and/or other purposes. The furnace wall 510 may further include additional or alternative types of heat transfer surface, such as refractory materials in some embodiments.

It will be recognized that the furnace wall 510 may include more than one gas burner 602, and that the gas burner(s) 602 may be disposed in one or more side walls of the furnace wall 510 as shown, in a bottom or floor surface (not shown) of the furnace wall 510, or at a top or ceiling surface (not shown) of the furnace wall 510.

According to an embodiment at least one electrode 616 is operatively coupled to the gas burner 602 and/or the flame 604. The at least one electrode 616 is configured to apply a high voltage or an electric field corresponding to the high voltage to or proximate to the flame 604. The electrode(s) 616 can be arranged substantially around a central axis of the gas burner 602 or may form a ring or cylinder concentric to the gas burner.

According to an embodiment, at least one electrical lead 618 is configured to supply the voltage to the at least one electrode 616, the at least one electrical lead 618 being operatively coupled to the gas burner 502. The at least one electrical lead 618 may be mechanically coupled to the outside of gas burner 602. For example, the at least one electrical lead 618 can be mechanically coupled to a refractory material 620 disposed along the outside of the gas burner 602. The at least one electrical lead 618 can alternatively be carried inside the refractory material 620 disposed along the outside of the gas burner 602. Alternatively, the at least one electrical lead 618 can be carried inside the gas burner 602, such as along the inside of a conduit, inside a non-conductive fuel delivery channel, and/or inside a combustion air delivery channel. The at least one electrical lead 618 can be cooled by a fluid flow (not shown) inside or peripheral to the at least one electrical lead 618.

According to an embodiment, a voltage source 622 (corresponding to the time-varying voltage source 116 described above) is configured to provide the voltage to the at least one electrode 616. The voltage source 622 can be configured to provide a time-varying voltage to the at least one electrode 616 such as a chopped direct current (DC) voltage, an alternating current (AC) voltage, or an AC voltage superimposed over a DC-bias voltage.

The voltage source 622 can be configured to apply a periodic voltage waveform to the at least one electrode 616. The periodic voltage waveform can be characterized by a frequency between 200 and 800 Hertz, for example. The periodic voltage waveform can include a high voltage waveform between 0-40,000 volts for, example. For the example of an AC waveform, the periodic voltage waveform can be ±2000 to ±100,000 volts, for example. Other frequencies and/or other voltages can be substituted without departing from the spirit or scope of the claims. The periodic voltage waveform may be a pulse width modulated (PWM) waveform, and the voltage source 622 may control rise, fall and other voltage level and timing elements of the waveform. The furnace wall 510 may be electrically grounded.

According to an embodiment, a control interface 624 is configured to control the voltage source 622. Functions and certain circuitry of control interface 624 are, for the time varying voltage source 116 above, described as being integrated with the voltage source 116. They may be so integrated, or may, as describe here, be implemented separately. The control interface 624 may affect any one or more of at least voltage level, phase, duty cycle, wave shape of the time varying voltage supplied to the voltage source 622.

The control interface 624 can control the voltage source 622 to maintain a temperature of water in steam tubes 516 to provide immunity from changes in fuel quality, to compensate for variations in fuel flow rate, to compensate for changes in environment, and/or or to minimize of one or more components of a flue gas that escapes the furnace 600, e.g., at the flue 520. In order to do so, the control interface may, in an embodiment, receive data from one or more sensors associated with the steam tubes. For example, the one or more sensors may provide temperature and/or pressure data to the control interface 624. The control interface 624 may be further configured to receive data associated with fuel and combustion flow rates, flame color temperature, and the like, from which the control interface may calculate emissivity and/or thermal radiation of the gas flame. The control interface 624 may be configured to change parameters of the time-varying voltage when the emissivity and/or thermal radiation of the gas flame do not fall within pre-determined threshold ranges. Such configuration to change voltage parameters may be stored in a memory device of the control interface 624 and executed by a processor of the control interface 624. The processor may include a microcontroller, field-programmable gate array (FPGA), application specific integrated circuit (ASIC), state machine, integrated circuit(s), and/or discrete circuitry to output a waveform.

FIG. 7 is a flow chart showing a method 701 for retrofitting a coal-fired furnace with the system for radiating energy from a flame. In replacing an existing burner system (e.g., burner system 501) in a furnace such as a coal-fired furnace 500, the existing burner system must be removed 702. In some instances, the existing burner system may include various attachment mechanisms (not illustrated) that mechanically fix the existing burner system in position relative to the furnace and/or to peripheral structures (e.g., floor, support scaffold, etc.) The attachment mechanisms may include, but are not limited to, one or more of bolts, plates, welds, buckles, shims, and the like. The existing burner system, including a burner (e.g., coal-fired burner 503) and fuel-and combustion air delivery device (e.g. pulverized coal and combustion air delivery device 507) must be detached at the attachment mechanisms, and the existing burner system removed. Replacement of the existing burner may also require replacement of a fuel supply, combustion air supply, and any control mechanisms and/or circuitry.

A gas-fired replacement burner system having the above-described flame charging system may be provided 704 to replace the removed coal burner.

The replacement gas burner system may be secured 706 proximate the furnace. The replacement gas burner system having the flame charging system may include attachment mechanisms corresponding, in location and/or type, to those of the pre-existing burner for each of attachment to the furnace and/or surrounding structure. For example, the replacement gas burner system may include tabs having (or drillable to have) holes for attaching the replacement gas burner system to the furnace and/or peripheral structure. Alternatively, the furnace and/or peripheral structures maybe modified to secure the replacement burner system, including attachment mechanisms for a replacement burner, fuel and combustion air devices, associated piping, and the like.

The method of replacing the existing coal burner with the replacement gas burner system may include placing refractory materials about a backside of the burner outside the furnace when the replacement gas burner system is installed. This refractory material may, for example, reduce heat loss from a back side of the burner and/or may insulate electrical leads (e.g., electrical leads 618) from the heat of the burner.

In some embodiments, the replacement gas burner system may be included in a coal-fired furnace retrofit kit. The kit may include a gas burner system, at least one electrically isolated conductor, and a voltage source. The gas burner system may be sized (geometrically and/or operationally) for direct or near-direct replacement of a coal burner in a coal-fired furnace. The gas burner system may include a gas burner, a fuel supply, and a combustion air supply. The gas burner may be configured to hold a gas flame. The fuel supply may convey a gaseous fuel to the burner, e.g., via a fuel nozzle to support combustion in a flame. The combustion air supply may convey combustion air, e.g., via blowers, swirl vanes, or the like, to support the combustion in a flame. The electrically isolated conductor may be operatively coupled to the gas burner. The conductor (e.g., electrode) may be placed near the gas burner such that it may (but need not) impinge with the gas flame. For example, the conductor may form a ring or short cylinder around a portion of where the flame is to be held. Alternatively, several conductors may be formed in proximity to the flame area. Such conductors may in some instances be electrically connected, e.g. via a single common electrical connection, or may each include its own electrical connection.

The electrically isolated conductor(s) may be configured to receive and supply a high voltage or electrical field to the gas flame. In accord with an embodiment the conductor(s) may operate together, or may operate in sequence, to provide a time varying high voltage waveform. The high-voltage waveform may be provided by a voltage source having circuitry to control the provision of the high-voltage waveform. The voltage source may include or cooperate with a control interface that controls at least one of the level, phase, and shape of the high-voltage waveform as described more particularly above.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

What is claimed is:
 1. A furnace, comprising: a furnace wall formed about at least a radiant heating volume and having a plurality of steam tubes disposed within the radiant heating volume; and a burner system disposed at the furnace wall and including: a gas burner disposed through an opening in the furnace wall and configured to hold a gas flame in the radiant heating volume, a gas and combustion air delivery device configured, respectively, to output a gas fuel and combustion air to the gas burner to support the gas flame, a flame charging system configured to impart a time-varying electrical charge onto the gas flame, and at least one electrode operatively coupled to the gas burner and disposed proximate the gas flame, the at least one electrode being disposed to apply an electric field to or proximate to the gas flame.
 2. The furnace of claim 1, wherein the burner system further comprises: a voltage source configured to provide a time-varying voltage to the flame charging system; and a control interface operably connected to the voltage source and configured to control at least one of a level, phase, duty cycle, and wave shape of the time-varying voltage.
 3. The furnace of claim 1, wherein the flame charging system of the burner system includes an energization electrode disposed to have at least intermittent contact with the gas flame and configured to receive the time-varying voltage.
 4. The furnace of claim 3, wherein the time-varying voltage is selected to cause the energization electrode of the flame charging system to impart the corresponding time-varying electrical charge onto the gas flame.
 5. The furnace of claim 1, wherein the flame charging system further comprises a charge-ejecting electrode disposed proximate the gas flame; and wherein the charge-ejecting electrode is configured to receive a time-varying electrical voltage and to eject a corresponding time-varying electrical charge toward the gas flame.
 6. The furnace of claim 5, wherein the charge-ejecting electrode includes a corona electrode.
 7. The furnace of claim 1 wherein the flame charging system includes an ionizer configured to receive a time-varying voltage and to provide a fluid medium carrying corresponding time-varying electrical charges to or in proximity to the flame.
 8. The furnace of claim 1, wherein the radiant heating volume is configured for receipt of heat from a coal burner, and wherein the burner system is a replacement burner system that replaces the coal burner, the flame charging system configuration to impart the time-varying electrical charge onto the gas flame is to increase emissivity of the gas flame.
 9. The furnace of claim 8, wherein the radiant heating volume is configured for receipt of heat from a coal burner, and wherein the burner system is a replacement burner system that replaces the coal burner, the flame charging system configuration to impart a time-varying reactant ratio into the gas flame is to increase emissivity of the gas flame.
 10. A coal-to-gas burner retrofit kit, comprising: a gas burner system sized to replace a coal burner in a furnace, the gas burner system including a fuel supply and a combustion air supply respectively configured to convey a gas fuel and combustion air, and a gas burner configured to hold a gas flame resulting from combustion of the gas fuel and combustion air received from the fuel supply and combustion air supply; at least one electrode operatively coupled to the gas burner and placed for impingement with the gas flame, the at least one electrode being disposed proximate a location for the gas flame and arranged to apply a time-varying high voltage to the gas flame; a voltage source having circuitry configured to control supply of the high voltage to the at least one electrode; and an electrically isolated electrode configured to interact with the time-varying high voltage to increase flame luminance.
 11. The coal-to-gas burner retrofit kit of claim 10, wherein the time-varying high voltage on the at least one electrode is selected to interact with the electrically isolated electrode to cause a time-varying position of the gas flame.
 12. The coal-to-gas burner retrofit kit of claim 11, wherein the time varying-position of the gas flame is selected to cause a time-varying reactant mixture.
 13. The coal-to-gas burner retrofit kit of claim 10, wherein the gas burner is sized to replace the coal burner in the furnace without modification of an opening of the furnace that receives the gas burner.
 14. The coal-to-gas burner retrofit kit of claim 10, further comprising one or more attachment mechanisms affixed to the gas burner system, the attachment mechanisms having attachment features that secure at least a portion of the gas burner system to the furnace when the gas burner system is installed to the furnace.
 15. The coal-to-gas burner retrofit kit of claim 14, wherein the attachment mechanisms are placed in correspondence with existing attachment features of the furnace.
 16. The coal-to-gas burner retrofit kit of claim 10, further comprising refractory material configured to be disposed along a portion of the gas burner that is outside the furnace when the gas burner system is installed to the furnace.
 17. A method for retrofitting a furnace, the method comprising: removing, from the furnace, an existing coal burner system, the existing coal burner system having a coal burner disposed at a burner opening of the furnace, a coal supply, and a combustion air supply; providing a replacement burner system for radiating energy from a gas flame, the replacement burner system including: a gas fuel source disposed to provide gaseous fuel via a fuel nozzle to the gas flame, a combustion air source disposed to provide combustion air to the gas flame, a gas burner configured to hold the gas flame produced from the gaseous fuel and the combustion air, a flame charging system configured to receive a time-varying voltage and to impart a corresponding time-varying electrical charge to the gas flame, and an electrically isolated conductor disposed proximate the gas burner; and securing the replacement burner system proximate the furnace, the gas burner disposed at the burner opening.
 18. The method for retrofitting a furnace of claim 17, wherein the replacement gas burner system further comprises: a voltage source configured to provide the time-varying voltage to the flame charging system; and a control interface operably connected to the voltage source and configured to control at least one of a voltage level, phase, duty cycle, and wave shape of the time-varying voltage.
 19. The method for retrofitting a furnace of claim 17, wherein the flame charging system of the replacement gas burner system further comprises an energization electrode disposed to have at least intermittent contact with the gas flame and configured to receive the time-varying voltage.
 20. The method for retrofitting a furnace of claim 19, wherein the time-varying charge is selected to cause the energization electrode to impart the corresponding time-varying electrical charge onto the gas flame.
 21. The method for retrofitting a furnace of claim 17, wherein the replacement gas burner system further comprises a charge-ejecting electrode disposed proximate the flame; and wherein the charge-ejecting electrode is configured to receive the time-varying electrical voltage and to eject a corresponding time-varying electrical charge toward the flame.
 22. The method for retrofitting a furnace of claim 17, wherein the replacement gas burner system further includes attachment features configured to attach the existing coal burner system to the furnace, the securing of the replacement gas burner system including attaching the replacement gas burner system to the furnace via the attachment features.
 23. The method for retrofitting a furnace of claim 17, wherein the replacement gas burner system further includes refractory material, the method further comprising installing the refractory material along a portion of the gas burner that is outside the furnace. 